I picked up When I Was a Child… with a curiosity about Marilynne Robinson equal only to her disinclination to give anything away – in a homespun, ordinary, autobiographical sense – about herself. This book is not, as its title might suggest, a memoir. Nor is it about childhood. She says there is a difference – for a writer as well as for readers – between "knowing" a person and "knowing about" them, and the extraordinary experience of reading these idiosyncratic, high-minded theological essays is about the former. It is the equivalent of an uncommon library ticket, an admission to the subjects that most obsess her: the frail human enterprise, faith and its absence, mysteries that elude language.

All Robinson's writing has a devotional quality. On the face of it, her novels – especially Gilead and its sequel Home – were unlikely successes because of their Christian hearts. Why would an unhurried book about a pastor named Reverend Ames disarm secular audiences (and win the Pulitzer prize)? Perhaps because, aside from the consummate delicacy with which these novels were written, they supplied a lack, were concerned with the sacred in a way much contemporary fiction is not.

Her novels have a unique atmosphere, a sad, steady sense of human faultiness, but there is, at the same time, always the possibility of grace. She says in this new book that she detects in many people now a "wistfulness and regret for the loss of Christianity."

Robinson teaches creative writing in Iowa City. She was raised a Presbyterian, became a Congregationalist and is a great defender of Calvinism (as in her previous essay collection The Death of Adam). In all her writing, her unrushed relationship with time – and to the past – is crucial (and I don't mean the 24-year gap between her first and second novels). This book is scholarly closework, as painstaking as a Victorian sampler but more subtle. She is determined never to undervalue or oversimplify. There is a sense that to be meditative is a necessary part of being alive. She is especially clear on the absurdity of seeing religion and science as adversarial. She urges fearful believers to "subscribe to Scientific American for a year" to extend their wonder at God's grandeur.

And she tells us, in passing, about a scientific experiment of her own. She was educated at a centre of behaviourist psychology where they "pestered" rats (in a maze-learning experiment) and attempted to "lure" them with Cheerios – her rat turned out not to be easy to bribe. It seems appropriate that her rat was a rebel for there is nothing Robinson likes less than facile conclusions about motivation. She deplores "so-called rational choice economics which assumes we will all find the shortest way to the reward". She has no truck with the selfish gene – if anything, she seems to think a selfless gene more likely. She asks why society is full of arrangements that "seem to inhibit or defeat self-interest?" Yet, at the same time, she is anti "austerity" as a policy (and is especially eloquent about its deleterious effects on American universities).

Resistance to explanation informs her writing too. Her characters evolve through intuition: "A character is really the sense of a character, embodied, attired and given voice as he or she seems to require."

She does not say much about the literature that has fed her imagination. Her childhood library does not appear to include any stalwarts – no mention of Lewis Carroll, CS Lewis, Anna Sewell. What she says instead is: "I preferred books that were old and thick and hard. I made vocabulary lists." As a child, she knew about "Constantinople, the Cromwell revolution and chivalry". She was a mini-scholar, and an imaginative one with a light self-mockery to leaven her earnestness. There is a beautiful sentence in a chapter describing her books, read and unread, where she imagines Homer coming to call: "It pleases me to think how astonished old Homer, whosoever he was, would be to find his epics on the shelf of such an unimaginable being as myself, in the middle of an unrumoured continent."

Reading turned her into a writer. And it stirs her to protest and defend. She champions the Old Testament, Moses and his laws, Johann Friedrich Oberlin, Iowa, American hymns and 18th-century theologians Charles Finney and Jonathan Edwards. Her adversaries meet a gentle nemesis: Bishop Spong, Jack Miles (author ofGod: a Biography), evolutionary psychologist Satoshi Kanazawa. Christopher Hitchens, Richard Dawkins and Sam Harris are swiftly dismissed for the "degree of consensus" in their books, "not what one would expect to find in the work of independent thinkers addressing the largest of arguments".

Her critical armoury includes no ostentatious weapons – no cruelty, sarcasm or rhetorical flourishes. She doesn't do polemic. Yet, as a non-believer, there were occasions when she was in the pulpit and I was on my knees.

There is a subtle tension throughout the book between society and solitude. She describes poetry as "a highly respectable use of solitude", the writings of Edgar Allan Poe have an "almost hallucinatory loneliness", and, she reveals – wonderful detail – that, in Iowa, the word "lonesome" has positive connotations. Yet what makes this book is Robinson's belief in community, her feeling that "it is in the nature of people to do good to one another" (as well as to sin), and that the world is, to quote Louis MacNeice, "incorrigibly plural" (even if, for her, it has only one God). Robinson is adept at studying the small print and reading between the lines but she never forgets to look up at the stars.